This application claims priority to German Patent Application No. 10 2004 059 985.8, filed on Dec. 13, 2004.
Transmitters are used for the transmission of information via wireless or wire line data traffic channels. To transmit information from different users through a single data channel, communication methods such as Frequency Division Multiple Access (FDMA) and Code Division Multiple Access (CDMA) can be used. Another method that allows for different information to be transmitted in different time slots is Time Division Multiple Access (TDMA) which is used in European telecommunication standards such as the Global System for Mobile Communications (GSM).
The power level of an output signal produced by a transmitter working in a TDMA system has to be ramped up or down according to a well defined ramping profile. In a GSM communication system, this profile is defined as a time dependent sequence of power level values such as described in the 3rd Generation Partnership Project (3GPP) specification TS 45.005. This standard also defines the allowed spectral densities of the output signal.
Another standard that is based on the GSM standard and that uses TDMA is the Enhanced Data GSM Environment (EDGE) standard. To transmit using standards such as EDGE, transmitter modulation units must modulate an output signal according to an 8-PSK (phase shift keying) scheme. The modulation of the output signal is a phase and amplitude binary coding of the transmitted information. The use of amplitude modulation with this approach requires that the transmitter include a linear transmission chain in order to avoid distorting the output signal.
One approach that can be used to ramp the output signal of a transmitter is to use a Variable Gain Amplifier (VGA). The VGA can be used to control the power level of signals such as a baseband signal or a high frequency signal modulated on a carrier frequency.
Another approach that can be used to control the power level of the output signal of a transmitter is a Programmable Gain Amplifier (PGA). With the PGA, a digital control word is set in order to adjust the gain of the PGA. The architecture of the transmitter for the PGA must be designed to allow for a continuous ramping of the power level of the output signal between a zero value and a target value in accordance with the standard being used.
In architectures that use a VGA or PGA, an output stage is used to convert a signal to a desired output power level before the signal is inserted into the transmission channel (e.g. by using an antenna). To achieve different output power levels, any amplification produced by elements within the transmitter signal path must be compensated by the VGA and/or the PGA. To provide this compensation, the amplifiers must be designed to have large dynamic ranges. For example, with GSM EDGE, the dynamic range has to be more than 40 dB in the high transmission band. With a VGA, this large dynamic range in combination with the spectral density and low noise requirements for the output signal results in high current consumption for the transmitter.